A Connecticut State Police officer runs with a shotgun following the shooting at Sandy Hook Elementary School in Newtown, Conn.

By Tracy Connor, NBC News

Police radio traffic from the Newtown school shooting shows emergency responders initially thought there might be two gunmen on the loose and were not aware of the extent of the carnage inside Sandy Hook Elementary School.

But the scope of the tragedy became more evident minute by minute, until authorities at the scene were heard asking for more help: “Call for everything” and “Do you know if anyone brought a mass casualty kit?”

Then, an hour after the first call, the horror of the crime was laid bare as an officer at the scene spoke of “victims” in a closet.

“There’s a teacher and 18 kids there,” he said in a grim voice.

The communications were not officially released, but were posted on YouTube by a scanner monitor and authenticated by police.

Some of the dialogue is encrypted or garbled, but the transmissions that can be heard – with the sound of sirens blaring in the background — provide a glimpse of how Friday’s massacre unfolded through the eyes of police and paramedics.

The recordings begin at 9:35 a.m. with a dispatcher calmly reporting a 911 call about “somebody shooting in the building,” followed two minutes later by the chilling update that a caller was “continuing to hear what he believes to be gunfire.”